1. Field of the Invention
The present invention pertains to a method for optimization of ventilator function aiming at that desired therapeutic goals should be reached. The optimization therefor is achieved on the basis of investigated physiological properties of the respiratory system.
2. Description of the Prior Art
Mechanical ventilation may cause lung damage. High pressures lead to damage denoted barotrauma. Ventilation at low pressures and volumes may lead to collapse and re-expansion of lung compartments during expirations and inspirations, respectively. This phenomenon may be denoted Recorex. Recorex may lead to lung trauma because of shear forces between zones of collapsed and aerated lung parenchyma. Studies of lung mechanics and adaptation of mechanical ventilation to lung physiology allow reduction of barotrauma and Recorex. The elastic pressure volume diagram, the Pel/V curve, can be recorded, for example with an electronically controlled ventilator.
With one or several Pel/V curves recorded under different circumstances one can judge whether the tidal volume, Vt, as a whole or in-part falls within the pressure/volume range within which barotrauma and Recorex are minimal. Under guidance of this information one may change the pattern of ventilation in such a way that an optimal P/V range is used to ventilate the lungs. One may for example increase the positive end expiratory pressure, PEEP, if the lung volume is to low. An alternative is to increase the frequency or to reduce the time for expiration in relation to the time for inspiration so that the lungs during expiration do not empty to a degree such that elastic forces exerted by the thoracic cage and the lungs are equilibrated. Thereby, it is accomplished that the alveolar pressure at the end of expiration is positive, which is denoted auto-PEEP. This implies that the lung volume is increased so that Recorex is avoided. If one rather would find that the pressure should be diminished during inspiration, one may decrease ventilation and thereby lower the peak airway pressure in order to decrease the risk for barotrauma.
Either an increase of PEEP or a lowering of the peak airway pressure frequently lead to a decrease of pulmonary gas exchange which may be deleterious. A lowering of the oxygenation of the blood can be counteracted by an increase in the fraction of oxygen inspired gas. This is, however, associated with risks. Another effect is that CO2-elimination is decreased which implies that carbon dioxide is retained in the body. Lately one has frequently accepted this effect, denoted permissive hypoventilation, however, without improved clinical results. Several physiological mechanisms are dependent on pH and thereby on the partial pressure of CO2 in arterial blood, PaCO2. Accordingly it is important, not only to exercise control over airway pressures but also over gas exchange so that PaCO2 is maintained within suitable limits.
By recording of a so called single breath test for CO2 one may estimate to what extent a change of Vt and minute ventilation, Vmin, will lead to a change of CO2 elimination expressed in ml/min. By measuring, or estimating, how much CO2 elimination changes at a change of breathing pattern one can during the following breaths estimate to what extent the change will lead to a change of PaCO2.
In order to improve the results of mechanical ventilation at grave lung disease both barotrauma and Recorex should be avoided. In order to get around that the gas exchange is unduly affected so that CO2 retention or hypoxia develops one may better use the Vt. by flushing the connecting tubes, particularly the tracheal tube with unspoiled gas during the later part of expiration. Thereby one may decrease the dead space so that one may decrease Vt without the risk for reduced gas exchange.
The pulmonary exchange of carbon dioxide and oxygen are coupled to one another because for each volume of oxygen taken up and consumed through metabolism a thereto proportional volume of carbon dioxide is produced and eliminated. What has been said above about carbon dioxide is accordingly paralleled by a corresponding phenomenon for oxygen. Technically it is more difficult to make fast and accurate determination of oxygen concentration than of carbon dioxide concentration. Particularly, at high inspired oxygen concentrations it is very difficult to accurately measure oxygen uptake. For these reasons carbon dioxide is below focussed upon in discussions about gas exchange although most aspects are relevant also with respect to oxygen.
Strategies for avoidance of lung damage following mechanical ventilation, and even to protect the lungs against damage, is denoted protective lung ventilation, PLV.
U.S. Pat. No. 4,917,080 desribes a method for controlling a ventilating apparatus. In a first simulator the characteristics of the ventilating apparatus is simulated and in a second simulator patient parameters are simulated. An adjustment to the ventilating apparatus can first be processed in the first simulator and the output is coupled to the second simulator to derive an effect of the adjustment as new patient data from the second simulator. The adjustment may then be switched to the ventilating apparatus or entered manually.
GB 2 093 218 discloses a respirator comprising a screen and controlled by a microprocessor. On the screen curves reflecting the result of new respiratory conditions can be shown. This simulation can be carried out while ventilation takes place with pre-established parameters. If the new values are satisfactory, the operator can request the change in the ventilation of the patient.
One objective of the present invention concerns new principles for determination of how PLV should be performed on the basis of the physiological characteristics of the individual patient and in consideration of the demands of an adequate gas exchange. The invention is founded on measurements of respiratory mechanics and also of gas exchange. The measurements are followed by a mathematical characterisation of these physiological characteristics. Thereafter a computer performs an analysis of the mode of ventilation by the ventilator with the goal that the therapeutic goal defined by the responsible operator (doctor or therapist) is reached. In a typical case the goal is defined as a combination of PLV and adequate gas exchange.
Methods for studies of the mechanics of the respiratory system are previously known, for example through the Swedish patent C2 506521 and Repiratory Mechanics During Mechanical Ventilation in Health and Disease,xe2x80x9d C. Svantesson, Thesis, Department of Clinical Physiology, Lund University (1997), ISBN 91-682-2766-9 and xe2x80x9cA Single Computer-Controlled Mechanical Insufflation Allows Determination of the Pressure-Volume Relationship of the Respiratory System,xe2x80x9d Svantesson et al., Journal of Clinial Monitoring and Computing (1999). Accordingly, one may use a computer-controlled ventilator to study the mechanics of the respiratory system. A pressure and volume range extending beyond the range of the tidal volume at the current setting of the ventilator, Vt, may be studied. A modification of one expiration by prolongation of the time for expiration and/or by reducing PEEP allows a volume range below the Vt to be studied. A volume range above the Vt may be studied by increasing of the volume of gas that is insufflated during the inspiration following the modified expiration.
The three segments of the sigmoid Pel/V curve are described by an equation in which the elastic recoil pressure is related to the volume, V. The reference volume for V is the lowest observed volume during the measurement. The intermediate segment which begins at the volume Vlip and the pressure Plip is linear and has, accordingly, a constant slope corresponding to a value of compliance denoted Clin. The lower and the upper segments are non-linear. The slopes of these segments (compliance) approach asymptotically zero when the curves are extrapolated towards a low and a high volume, denoted Vmin and Vmax, respectively.
The equation that describes Pel as a function of V can be as follows:
For V less than Vlip:
Pel=Plipxe2x88x92(Vlipxe2x88x92Vmin)/Clin*ln((Vminxe2x88x92Vlip)/(Vminxe2x88x92V))
For Vlipxe2x89xa6Vxe2x89xa6Vuip:
Pel=Plip+(Vxe2x88x92Vlip)/Clin
For V greater than Vuip:
Pel=Puip+(Vmaxxe2x88x92Vuip)/Clin*ln((Vmaxxe2x88x92Vuip)/(Vmaxxe2x88x92V))xe2x80x83xe2x80x83(1)
The pressure that drives flow through the airways can in analogy with Ohm""s law be calculated as resistance multiplied by flow rate. Inspiratory resistance, R(i), often varies to a small extent during the breath. It may decrease with increasing lung volume. The pressure that drives flow during insipiration Pres(i), can defined as a function of flow rate, F, and volume, V:
Pres(i)=(r0+r1*V)*Fxe2x80x83xe2x80x83(2)
r0 denotes the resistance at zero volume, while r1, usually a negative number, gives the variation of R(i) with volume.
Equation 1 and 2 describe the mechanical behaviour of the respiratory system during inspiration under various circumstances. Their coefficients can be determined with known methods. However, simpler equations may often be used. Within a limited volume range a linear equation may with adequate accuracy escribe the Pel/V curve. The inspiratory resistance can often be represented by a constant. This invention can be embodied on the basis of other equations, which in the actual situation adequately represent the elastic and resistive properties of the respiratory system. Furthermore, the invention is not dependent upon which method is used for measurement and analysis leading to a mathematical description of the elastic and resistive properties of the respiratory system.
The above object is achieved in accordance with the principles of the present invention in a first embodiment of a method and an apparatus for searching for an optimal mode of ventilation, from among a number of ventilation modes in a ventilator, wherein measuring parameters related to the properties of the respiratory system are obtained, a mathematical description of the respiratory system is formed based on the measured parameters, wherein mathematical descriptions for different modes of ventilation are also formed, wherein the mathematical descriptions are tested for different modes of ventilation using the mathematical description of the respiratory system, in order to determine simulated results, the simulated results are compared with a therapeutic goal, and the mode of ventilation is identified which provides a simulated result that comes closest to the therapeutic goal, the identified ventilation mode being considered the optimal mode.
In a further embodiment of the method and apparatus, instead of comparing the simulated results to the therapeutic goal, an iterative analysis is undertaken by forming a mathematical description of a new, different mode ofventilation, testing the new mathematical description for the new, different mode of ventilation on the mathematical description of the respiratory system to determine a new simulated result for the different mode of ventilation, and the new mathematical description for each new, different mode of ventilation is based on the therapeutic goal, a comparison between the simulated result and the therapeutic goal for previously applied mathematical descriptions, and predetermined limits for changes in parameters between two consecutive new modes of ventilation. Again, the mode of ventilation that provides a simulated result coming closest to a therapeutic goal is selected as the optimal mode.